System and method for joint time tracking of multiple paths

ABSTRACT

In a receiver having at least two fingers, the fingers tracking at least one path of a multipath channel, a method includes the steps of forming a finger block of at least two of the two fingers and jointly tracking the fingers of said finger block. The step of jointly tracking includes the steps of generating direction metrics of each of a set of possible directions of joint movement of the fingers of the finger block, selecting one of the direction metrics according to a predetermined criterion, and moving the fingers of the finger block in the directions indicated by the selected direction metric.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a Divisional Application from U.S. patentapplication Ser. No. 09/301,116, filed Apr. 28, 1999, now issued U.S.Pat. No. 6,314,130.

FIELD OF THE INVENTION

The present invention relates to systems and methods for tracking thedelays of channel paths in general and of jointly tracking the timedelays of multiple paths (multipath components) in particular.

BACKGROUND OF THE INVENTION

In recent years, direct sequence (DS) code division multiple access(CDMA) spread spectrum communication systems and methods haveexperienced growing attention worldwide. The IS-95 cellularcommunication standard is one example of an application of DS-CDMAcommunications, as described in the article TIA/EIA/IS-95-A, “MobileStation-Base Station Compatibility Standard for Dual-Mode WidebandSpread Spectrum Cellular System”, Feb. 27, 1996. Additional applicationsinclude third generation cellular systems, wireless multimedia systems,personal satellite mobile systems, and more.

In DS-CDMA communications, each user is assigned a distinct spreadingcode often referred to as pseudo noise (PN) sequence. The spreading codebits (called chips) are used to modulate the user data. The number ofchips used to modulate one data symbol is known as the spreading factorof the system, and it is related to the spreading in bandwidth betweenthe (unmodulated) user data and the CDMA signal. In this simplest form,the baseband equivalent of the transmitted CDMA signal, sampled at thechip rate 1/Tc, is${{T\lbrack n\rbrack} = {\sum\limits_{\mathcal{L} = 1}^{K}{{{a_{1}\left\lbrack \left\lfloor {{n/S}\quad F} \right\rfloor \right\rbrack} \cdot P}\quad {N_{i}\lbrack n\rbrack}}}},$

where Tc is the chip duration, └x┘ denotes the integer part of x, SF isthe spreading factor, a_(i)[└n/SF┘] and PN_(i)[n] are the data symboland spreading code of the i-th user, respectively, and K is the numberof active users. Note that by the definition of └x┘, a_(i)[└n/SF┘] isfixed for SF consecutive chips, in accordance with the definition abovethat each data symbol is modulated by SF chips.

An important feature of DS-CDMA systems is that they provide thepossibility of obtaining excellent immunity to multipath fading throughresolving the individual, time separated multipath components andoptimally combining them. The common approach for achieving this is touse a “rake” receiver as is known in the art. Such a receiver assignsdespreading correlators to each of the dominant multipath components andsynchronizes them for maximum de-spread power. For each of the rake“fingers”, the phase and amplitude of the corresponding channelmultipath component is estimated and used to apply amplitude weightingand phase alignment prior to combining. The weighted sum of themultipath components will experience considerably less fading than anyof the individual components so that a diversity gain is obtained.

As is known in the art, a crucial requirement of the rake receiver isthat its fingers are time aligned (synchronized) with the multipathcomponents of the channel. This requires estimation of the multipathdelays and is often achieved by a simple early-late time trackingmechanism. The early-late mechanism is, in fact, a delay-lock-loop thatmeasures the energy prior (early) and after (late) the current samplinginstances. These early and late energy measurements are used to lock onthe sampling instance that maximizes the sampled signal energy. As itturns out, these maximal energy sampling instances leads, in many cases,to the desired synchronization of the rake fingers to the channelmultipath components. However, some channels, for example thoseencountered in dense urban environments, consist of a large number ofclosely spaced multipath components. This leads to multipath clustersthat are often spaced less than Tc apart. Conventional early-late timetracking mechanism are often incapable of tracking the delays associatedwith those closely spaced multipath clusters since their early and latemeasures are a superposition of the energies associated with severaladjacent clusters. In such a situation, the rake fingers are notproperly time aligned with the multipath clusters, leading todegradation in the receiver performance.

It would therefore be beneficial to have an improved time trackingmechanism that is more robust to the presence of closely spacedmultipath components.

It would also be beneficial to have an improved criterion for fingerassignment in closely spaced multipath environment.

In recent years several methods for combating closely spaced multipathcomponents in DS-CDMA communication systems were derived. In U.S. Pat.No. 5,692,006 to Ross, U.S. Pat. No. 5,648,983 to Kostic et al. and U.S.Pat. No. 5,793,796 to Hulbert et al. it is suggested to avoid directestimation of the path delays. Instead, a bank of closely spaced fingersis utilized to effectively cover a pre-specified delay window. Thusinstead of actually estimating the multipath delays, all possible delaysin the window are examined and weighted according to some qualitymeasure criterion. In U.S. Pat. No. 5,692,006 a conventional LMSalgorithm is used to adaptively estimate of optimal finger weighting,whereas in U.S. Pat. No. 5,648,983 a weighted least squares solution isused to assign the finger weights.

Other solutions can be found in:

EP Patent Publication 704 985 A2 to Hulbert;

U.S. Pat. No. 5,764,688 to Hulbert et al.;

L. Dumont, et al., “Super-resolution of Multipath Channels in a SpreadSpectrum Location System”, Electronics Letters, Vol. 30, No. 19, Sep.15, 1994; and

Makoto Takeuchi et al., “A Delay Lock Loop Using Delay Path Cancellationfor Mobile Communications”, Electronics and Communication in Japan, Part1, Vol. 79, No. 4, 1996.

SUMMARY OF THE INVENTION

The present invention provides an improved time tracking mechanism thatis more robust to the presence of closely spaced multipath components.

The present invention also provides a criterion for finger assignment ina closely spaced multipath environment.

There is therefore provided in accordance with a preferred embodiment ofthe present invention a method used in a receiver having at least twofingers forming a finger block, the finger block tracking at least onepath of a multipath channel. The method includes the steps of generatingdirection metrics of each of a set of possible directions of jointmovement of the fingers of the finger block, selecting one of thedirection metrics according to a predetermined criterion, and moving thefingers of the finger block in the directions indicated by the selecteddirection metric.

Moreover, in accordance with a preferred embodiment of the presentinvention, the selected direction metric is the maximal directionmetric.

Furthermore, in accordance with a preferred embodiment of the presentinvention, the step of moving adjusts the fingers of the finger blockonly if the selected direction metric is the maximal direction metricand exceeds a comparison direction metric by at least a predeterminedthreshold.

Additionally, in accordance with a preferred embodiment of the presentinvention, the method further includes the step of redefining fingerblocks after the step of moving.

Moreover, in accordance with a preferred embodiment of the presentinvention, the finger block is formed of two fingers.

Furthermore, in accordance with a preferred embodiment of the presentinvention, the direction metrics are generated for five, six or ninedifferent directions of joint movement.

Additionally, in accordance with a preferred embodiment of the presentinvention, the finger block is formed of two closely spaced fingers.

Moreover, in accordance with a preferred embodiment of the presentinvention, the closely spaced fingers are ⅞ chip apart.

Furthermore, in accordance with a preferred embodiment of the presentinvention, the finger block is formed of three fingers.

Additionally, in accordance with a preferred embodiment of the presentinvention, delays between fingers are set to be no smaller than ⅞ chip.

Moreover, in accordance with a preferred embodiment of the presentinvention, the step of generating includes the step of time averagingthe direction metrics by summing consecutive direction metrics.

Furthermore, in accordance with a preferred embodiment of the presentinvention, the step of time averaging uses a forgetting factor.

Additionally, in accordance with a preferred embodiment of the presentinvention, the direction metrics are based on power estimation.

There is also provided in accordance with a preferred embodiment of thepresent invention a method for deactivating a selected one of at leasttwo fingers forming a finger block in a receiver. The method includesthe steps of measuring powers of the fingers of the finger block,calculating crosscorrelations of the fingers of the finger block withone another, and deactivating the selected finger when a function of thepowers and the crosscorrelations satisfies a predetermined criterion.

Moreover, in accordance with a preferred embodiment of the presentinvention, the finger block has two fingers.

Furthermore, in accordance with a preferred embodiment of the presentinvention, the function is

FingerCost=α·min(PowerCenter₁, PowerCenter₂)−ρ·C

where α is a value between zero and one, PowerCenter₁ and PowerCenter₂are powers of fingers in the finger block, C is a predetermined valueand ρ is one of the calculated crosscorrelations.

Additionally, in accordance with a preferred embodiment of the presentinvention, ρ is$\rho = \frac{{Re}{\sum\limits_{k = 1}^{N}\left( {{{ch}_{Finger1}(k)} \cdot {{ch}_{Finger2}^{*}(k)}} \right)}}{\sum\limits_{k = 1}^{N}\left| \left( {{{ch}_{Finger1}(k)} \cdot {{ch}_{Finger2}^{*}(k)}} \right) \right|}$or$\rho = {\frac{\left| {\overset{N}{\sum\limits_{k = 1}}{{{ch}_{Finger1}(k)} \cdot {{ch}_{Finger2}^{*}(k)}}} \right|}{\sqrt{\sum\limits_{k = 1}^{N}\left| {{ch}_{Finger1}(k)} \middle| {}_{2}{\cdot \sum\limits_{k = 1}^{n}} \middle| {{ch}_{Finger2}^{*}(k)} \right|^{2}}}.}$

Moreover, in accordance with a preferred embodiment of the presentinvention, the step of calculating uses a forgetting factor.

There is also provided in accordance with a preferred embodiment of isthe present invention a method for assigning a new path to a fingercandidate which is one of at least one inactive fingers in a receiveralso having a plurality of active fingers. The method includes the stepsof measuring the power of the new path, and calculating thecrosscorrelation of the finger candidate and the one of the activefingers whose time delay is closest to the time delay of the fingercandidate.

Moreover, in accordance with a preferred embodiment of the presentinvention, the method further includes the step of assigning the fingercandidate to the new path when a composite finger power for the fingercandidate satisfies a predetermined criterion.

Furthermore, in accordance with a preferred embodiment of the presentinvention, the step of calculating uses a crosscorrelation functiondefined as$\rho = \frac{{Re}{\sum\limits_{k = 1}^{N}\left( {{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}} \right)}}{\sum\limits_{k = 1}^{N}\left| \left( {{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}} \right) \right|}$or$\rho = {\frac{\left| {\overset{N}{\sum\limits_{k = 1}}{{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}}} \right|}{\sqrt{\sum\limits_{k = 1}^{N}\left| {{ch}_{FingerCandidate}(k)} \middle| {}_{2}{\cdot \sum\limits_{k = 1}^{n}} \middle| {{ch}_{ClosestFinger}^{*}(k)} \right|^{2}}}.}$

Additionally, in accordance with a preferred embodiment of the presentinvention, the step of calculating uses a forgetting factor.

There is also provided in accordance with a preferred embodiment of thepresent invention a method for reassigning a finger candidate which isone of a plurality of active fingers in a receiver to a new path. Themethod includes the steps of measuring the power of the new path andcalculating the crosscorrelation of the finger candidate and the one ofthe active fingers whose time delay is closest to the time delay of thefinger candidate.

Moreover, in accordance with a preferred embodiment of the presentinvention, the method further includes the step of reassigning thefinger candidate to the new path when a composite finger power for thefinger candidate exceeds the minimum composite finger power of all theactive fingers.

Furthermore, in accordance with a preferred embodiment of the presentinvention, the step of calculating uses a crosscorrelation functiondefined as$\rho = \frac{{Re}{\sum\limits_{k = 1}^{N}\left( {{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}} \right)}}{\sum\limits_{k = 1}^{N}\left| \left( {{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}} \right) \right|}$or$\rho = {\frac{\left| {\overset{N}{\sum\limits_{k = 1}}{{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}}} \right|}{\sqrt{\sum\limits_{k = 1}^{N}\left| {{ch}_{FingerCandidate}(k)} \middle| {}_{2}{\cdot \sum\limits_{k = 1}^{n}} \middle| {{ch}_{ClosestFinger}^{*}(k)} \right|^{2}}}.}$

Additionally in accordance with a preferred embodiment of the presentinvention, the step of calculating uses a forgetting factor.

There is also provided in accordance with a preferred embodiment of thepresent invention a receiver having at least two fingers forming afinger block, the finger block tracking at least one path of a multipathchannel. The receiver includes a direction metric determiner, a metricselector and a finger adjuster. The direction metric determinergenerates direction metrics of each of a set of possible directions ofjoint movement of the fingers of the finger block. The metric selectorselects one of the direction metrics according to a predeterminedcriterion. The finger adjuster moves the fingers of the finger block inthe directions indicated by the selected direction metric.

Moreover, in accordance with a preferred embodiment of the presentinvention, the selected direction metric is the maximal directionmetric.

Furthermore, in accordance with a preferred embodiment of the presentinvention, the finger adjuster moves the fingers of the finger blockonly if the selected direction metric is the maximal direction metricand exceeds a comparison direction metric by at least a predeterminedthreshold.

Additionally, in accordance with a preferred embodiment of the presentinvention, the finger adjuster includes a redefiner which redefinesfinger blocks once the fingers have been moved.

Moreover, in accordance with a preferred embodiment of the presentinvention, the finger block is formed of two fingers.

Furthermore, in accordance with a preferred embodiment of the presentinvention, the determiner generates the direction metrics for five, sixor nine different directions of joint movement.

Additionally, in accordance with a preferred embodiment of the presentinvention, the finger block is formed of two closely spaced fingers.

Moreover, in accordance with a preferred embodiment of the presentinvention, the closely spaced fingers are ⅞ chip apart.

Furthermore, in accordance with a preferred embodiment of the presentinvention, the finger block is formed of three fingers.

Additionally, in accordance with a preferred embodiment of the presentinvention, delays between fingers are set to be no smaller than ⅞ chip.

Moreover, in accordance with a preferred embodiment of the presentinvention, the direction metrics are based on power estimation.

There is also provided in accordance with a preferred embodiment of thepresent invention a finger deactivator for deactivating a selected oneof at least two fingers which form a finger block in a receiver. Thefinger deactivator includes a finger power measurement unit formeasuring the powers of the fingers of the finger block, a correlationcalculator for calculating the crosscorrelations of the fingers of thefinger block with one another, and a deactivation unit for deactivatingthe selected finger when a function of the powers and thecrosscorrelations satisfies a predetermined criterion.

Moreover, in accordance with a preferred embodiment of the presentinvention, the finger block has two fingers and the function is

FingerCost=α·min(PowerCenter₁, PowerCenter₂)−ρ·C

where α is a value between zero and one, PowerCenter₁ and PowerCenter₂are powers of fingers in the finger block, C is a predetermined valueand ρ is one of the calculated crosscorrelations.

Furthermore, in accordance with a preferred embodiment of the presentinvention, ρ is$\rho = \frac{{Re}{\sum\limits_{k = 1}^{N}\left( {{{ch}_{Finger1}(k)} \cdot {{ch}_{Finger2}^{*}(k)}} \right)}}{\sum\limits_{k = 1}^{N}\left| \left( {{{ch}_{Finger1}(k)} \cdot {{ch}_{Finger2}^{*}(k)}} \right) \right|}$or$\rho = {\frac{\left| {\overset{N}{\sum\limits_{k = 1}}{{{ch}_{Finger1}(k)} \cdot {{ch}_{Finger2}^{*}(k)}}} \right|}{\sqrt{\sum\limits_{k = 1}^{N}\left| {{ch}_{Finger1}(k)} \middle| {}_{2}{\cdot \sum\limits_{k = 1}^{n}} \middle| {{ch}_{Finger2}^{*}(k)} \right|^{2}}}.}$

Additionally, in accordance with a preferred embodiment of the presentinvention, the correlation calculator uses a forgetting factor.

There is also provided in accordance with a preferred embodiment of thepresent invention a finger assignor in a receiver having a plurality ofactive fingers and at least one inactive finger. The finger assignor isoperative to assign a finger candidate which is one of the at least oneinactive finger to a new path. The finger assignor includes a powermeasurement unit for measuring the power of the new path, and acorrelation calculator for calculating the crosscorrelation of thefinger candidate and the one of the active fingers whose time delay isclosest to the time delay of the finger candidate.

Moreover, in accordance with a preferred embodiment of the presentinvention, the finger assignor further includes an assignment unit forassigning the finger candidate to the new path when a composite fingerpower for the finger candidate satisfies a predetermined criterion.

Furthermore, in accordance with a preferred embodiment of the presentinvention, the correlation calculator uses a crosscorrelation functiondefined as$\rho = \frac{{Re}{\sum\limits_{k = 1}^{N}\left( {{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}} \right)}}{\sum\limits_{k = 1}^{N}\left| \left( {{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}} \right) \right|}$or$\rho = {\frac{\left| {\overset{N}{\sum\limits_{k = 1}}{{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}}} \right|}{\sqrt{\sum\limits_{k = 1}^{N}\left| {{ch}_{FingerCandidate}(k)} \middle| {}_{2}{\cdot \sum\limits_{k = 1}^{n}} \middle| {{ch}_{ClosestFinger}^{*}(k)} \right|^{2}}}.}$

Additionally, in accordance with a preferred embodiment of the presentinvention, the correlation calculator uses a forgetting factor.

There is also provided in accordance with a preferred embodiment of thepresent invention a finger assignor in a receiver having a plurality ofactive fingers. The finger assignor is operative to assign a fingercandidate which is one of the active fingers to a new path. The fingerassignor includes a power measurement unit for measuring the power ofthe new path, and a correlation calculator for calculating thecrosscorrelation of the finger candidate and the one of the activefingers whose time delay is closest to the time delay of the fingercandidate.

Moreover, in accordance with a preferred embodiment of the presentinvention, the finger assignor further includes an assignment unit forreassigning the finger candidate to the new path when a composite fingerpower for the finger candidate exceeds the minimum composite fingerpower of all the active fingers.

Furthermore, in accordance with a preferred embodiment of the presentinvention, the correlation calculator uses a crosscorrelation functiondefined as$\rho = \frac{{Re}{\sum\limits_{k = 1}^{N}\left( {{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}} \right)}}{\sum\limits_{k = 1}^{N}\left| \left( {{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}} \right) \right|}$or$\rho = {\frac{\left| {\overset{N}{\sum\limits_{k = 1}}{{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}}} \right|}{\sqrt{\sum\limits_{k = 1}^{N}\left| {{ch}_{FingerCandidate}(k)} \middle| {}_{2}{\cdot \sum\limits_{k = 1}^{n}} \middle| {{ch}_{ClosestFinger}^{*}(k)} \right|^{2}}}.}$

Additionally, in accordance with a preferred embodiment of the presentinvention, the correlation calculator uses a forgetting factor.

There is also provided in accordance with a preferred embodiment of thepresent invention a method used in a receiver having at least twofingers, the fingers tracking at least one path of a multipath channel.The method includes the steps of forming a finger block of at least twoof the at least two fingers, and jointly tracking the fingers of thefinger block.

Moreover, in accordance with a preferred embodiment of the presentinvention, the step of jointly tracking includes the steps of generatingdirection metrics of each of a set of possible directions of jointmovement of the fingers of the finger block, selecting one of thedirection metrics according to a predetermined criterion, and moving thefingers of the finger block in the directions indicated by the selecteddirection metric.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description taken in conjunction with theappended drawings in which:

FIG. 1 is a schematic illustration of the nine possible joint movementsof two fingers;

FIGS. 2A-2C are schematic illustrations of the twenty-seven possiblejoint movements of three fingers;

FIG. 3 is a flow chart illustration of a general method of jointly timetracking multiple paths, constructed and operative in accordance with apreferred embodiment of the present invention;

FIG. 4 is a schematic illustration of six possible joint movements,useful when considering only closely spaced fingers;

FIG. 5 is a flow chart illustration of the operation of a finger blockof two fingers, constructed and operative in accordance with a furtherpreferred embodiment of the present invention, and considering the jointmovements of FIG. 4;

FIG. 6 is a flow chart illustration of a method of redefining fingerblocks forming part of the method of FIG. 5;

FIG. 7 is a flow chart illustration of a method of finger addition andfinger replacing mechanism useful in a rake receiver performing thejoint tracking method of FIG. 3; and

FIG. 8 is a flow chart illustration of a method of switching off jointtracking, useful in a rake receiver performing the joint tracking methodof FIG. 5.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention jointly tracks the timing of multiple paths ormultiple path clusters. Any group of fingers, which jointly trackmultiple paths, is herein called a “finger block”. Finger blocks areespecially useful for fingers that track paths which are “close” totheir neighbors, but can also be used for paths which are notparticularly close.

The definition of “close path” can be any desired definition buttypically it defines paths which interfere with one another. Forexample, two close paths might be those whose time separation is lessthan 1.5 Tc, where Tc is the chip duration. “Close fingers” of a fingerblock are those which are within a certain predefined range RANGE ofdelays of its neighbors in the block, such as between one chip and halfa chip. Thus, for a finger block of two fingers, finger 1 and finger 2are within RANGE of each other. For a finger block of four fingers,where the fingers are ordered by their sampling point as finger 1,finger 2, finger 3 and finger 4, there is a delay within RANGE betweenneighboring fingers (finger 1 to finger 2, finger 2 to finger 3 andfinger 3 to finger 4).

FIG. 1, to which reference is now made, illustrates the possibilities toconsider for a finger block having two fingers, where each finger isshown with an arrow. Since the paths being tracked move about in time,the receiver must determine how the paths have moved in order to movetheir associated fingers.

As shown in FIG. 1, the possible nine movements are: both become later(direction 1), both become earlier (direction 2), both stay, the same(direction 3), the second becomes later (direction 4) or earlier(direction 9), the first becomes later (direction 6) or earlier(direction 5), and both move towards each other (direction 8) or awayfrom each other (direction 7).

FIGS. 2A-2C, to which reference is now briefly made, illustrate thepossibilities for a finger block of three fingers. There are 27 possiblefinger movements, all of which can be separately considered whenperforming a joint tracking operation. FIGS. 2A-2C are believed to beself-explanatory considering the previous discussion and therefore, willnot be further described.

To measure the possible movements, each finger is examined at itsnominal (or center) sampling time and at plus or minus one or more fixeddelays. The delays define “directions” of possible movement from thecenter sampling time. For example, a single delay, which might be ofTc/4, creates three directions of movement, “center”, “early” and“late”. The delayed signals are used to determine which of the possiblemovements actually occurred and to move the sampling point accordingly.

In the present invention, direction metrics are assigned to each of thepossible movement directions, using measurements of the delayed versionsof the data of the fingers of the finger block, and a decision tree isprovided to determine from the direction metrics which movementoccurred. Typically, the fingers are delayed slightly early and slightlylate, therefore each finger should be examined early, late and center(i.e. no delay). However, other types and amounts of delays are possibleand are incorporated into the present invention.

FIG. 3, to which reference is now made, is a flow chart illustration ofthe present invention. Initially, in step 10, a metric is assigned toeach of the possible joint movement directions. In step 12 the directionwith the strongest metric is chosen. If no finger movement is required(as in for example direction 3 of FIG. 1), then once again steps 10 and12 are performed. If a finger movement is required, (e.g. all directionsof FIG. 1 excluding direction 3), then each finger need to be moved ismoved to its new location in step 14. Finally, in step 16, aredefinition of the finger blocks occurs whenever the finger movement ofstep 14 causes new fingers to became within RANGE of each other, or whenclosely spaced fingers are separated and are no longer within RANGE ofeach other.

In one preferred embodiment step 10 is performed by assigningpower-based metrics to each of the possible joint direction metrics. Inaccordance with FIG. 1, the direction metrics are assigned to each ofthe 9 possible joint directions: $\left\{ \begin{matrix}{\quad {{{Direction}_{1}{\_ metric}} = {{PowerLate}_{1} + {PowerLate}_{2}}}} \\{\quad {{{Direction}_{2}{\_ metric}} = {{PowerEarly}_{1} + {PowerEarly}_{2}}}} \\{\quad {{{Direction}_{3}{\_ metric}} = {{PowerCenter}_{1} + {PowerCenter}_{2}}}} \\{\quad {{{Direction}_{4}{\_ metric}} = {{PowerCenter}_{1} + {PowerLate}_{2}}}} \\{\quad {{{Direction}_{5}{\_ metric}} = {{PowerEarly}_{1} + {PowerCenter}_{2}}}} \\{\quad {{{Direction}_{6}{\_ metric}} = {{PowerLate}_{1} + {PowerCenter}_{2}}}} \\{\quad {{{Direction}_{7}{\_ metric}} = {{PowerEarly}_{1} + {PowerLate}_{2}}}} \\{\quad {{{Direction}_{8}{\_ metric}} = {{PowerLate}_{1} + {PowerEarly}_{2}}}} \\{\quad {{{Direction}_{9}{\_ metric}} = {{PowerCenter}_{1} + {PowerEarly}_{2}}}}\end{matrix} \right.$

where PowerEarly_(i), PowerCenter_(i) and PowerLate_(i) are estimatedearly, center and late powers of finger_(i) respectively.

In another preferred embodiment, directions 6, 8 and 9 can be limited,so that if the delay between the fingers is less than a prespecifiedthreshold (e.g. ⅞ Tc), then the corresponding direction metric iszeroed. This ensures that those directions are not chosen in step 12 andprevents the situation where the fingers become too closely spaced.

Similarly, in a 3-finger block, direction metrics are assigned to allpossible 27 joint movement directions in accordance with FIGS. 2A-2C.

In another preferred embodiment, time averaging with or withoutforgetting factors (known in the art) can be applied to each of thedirection metrics in order to reduce their statistical variability Inone preferred embodiment Equations (1) are replaced by $\begin{matrix}\left\{ \begin{matrix}{{{Smoothed\_ L}_{1}{\_ L}_{2}} = \begin{matrix}{{\left( {1 - \alpha} \right){Smoothed\_ L}_{1}{\_ L}_{2}} +} \\{\alpha \left\lbrack {{PowerLate}_{1} + {PowerLate}_{2}} \right\rbrack}\end{matrix}} \\{{{Smoothed\_ E}_{1}{\_ E}_{2}} = \begin{matrix}{{\left( {1 - \alpha} \right){Smoothed\_ E}_{1}{\_ E}_{2}} +} \\{\alpha \left\lbrack {{PowerEarly}_{1} + {PowerEarly}_{2}} \right\rbrack}\end{matrix}} \\{{{Smoothed\_ C}_{1}{\_ C}_{2}} = \begin{matrix}{{\left( {1 - \alpha} \right){Smoothed\_ C}_{1}{\_ C}_{2}} +} \\{\alpha \left\lbrack {{PowerCenter}_{1} + {PowerCenter}_{2}} \right\rbrack}\end{matrix}} \\{{{Smoothed\_ C}_{1}{\_ L}_{2}} = \begin{matrix}{{\left( {1 - \alpha} \right){Smoothed\_ C}_{1}{\_ L}_{2}} +} \\{\alpha \left\lbrack {{PowerCenter}_{1} + {PowerLate}_{2}} \right\rbrack}\end{matrix}} \\{{{Smoothed\_ E}_{1}{\_ C}_{2}} = \begin{matrix}{{\left( {1 - \alpha} \right){Smoothed\_ E}_{1}{\_ C}_{2}} +} \\{\alpha \left\lbrack {{PowerEarly}_{1} + {PowerCenter}_{2}} \right\rbrack}\end{matrix}} \\{{{Smoothed\_ L}_{1}{\_ C}_{2}} = \begin{matrix}{{\left( {1 - \alpha} \right){Smoothed\_ L}_{1}{\_ C}_{2}} +} \\{\alpha \left\lbrack {{PowerLate}_{1} + {Power\_ Center}_{2}} \right\rbrack}\end{matrix}} \\{{{Smoothed\_ E}_{1}{\_ L}_{2}} = \begin{matrix}{{\left( {1 - \alpha} \right){Smoothed\_ E}_{1}{\_ L}_{2}} +} \\{\alpha \left\lbrack {{PowerEarly}_{1} + {PowerLate}_{2}} \right\rbrack}\end{matrix}} \\{{{Smoothed\_ L}_{1}{\_ E}_{2}} = \begin{matrix}{{\left( {1 - \alpha} \right){Smoothed\_ L}_{1}{\_ E}_{2}} +} \\{\alpha \left\lbrack {{PowerLate}_{1} + {PowerEarly}_{2}} \right\rbrack}\end{matrix}} \\{{{Smoothed\_ C}_{1}{\_ E}_{2}} = \begin{matrix}{{\left( {1 - \alpha} \right){Smoothed\_ C}_{1}{\_ E}_{2}} +} \\{\alpha \left\lbrack {{PowerCenter}_{1} + {PowerEarly}_{2}} \right\rbrack}\end{matrix}}\end{matrix} \right. & \text{(2A)} \\\left\{ \begin{matrix}{\quad {{{Direction}_{1}{\_ Metric}} = {{Smoothed\_ L}_{1}{\_ L}_{2}}}} \\{\quad {{{Direction}_{2}{\_ Metric}} = {{Smoothed\_ E}_{1}{\_ E}_{2}}}} \\{\quad {{{Direction}_{3}{\_ Metric}} = {{Smoothed\_ C}_{1}{\_ C}_{2}}}} \\{\quad {{{Direction}_{4}{\_ Metric}} = {{Smoothed\_ C}_{1}{\_ L}_{2}}}} \\{\quad {{{Direction}_{5}{\_ Metric}} = {{Smoothed\_ E}_{1}{\_ C}_{2}}}} \\{\quad {{{Direction}_{6}{\_ Metric}} = {{Smoothed\_ L}_{1}{\_ C}_{2}}}} \\{\quad {{{Direction}_{7}{\_ Metric}} = {{Smoothed\_ E}_{1}{\_ L}_{2}}}} \\{\quad {{{Direction}_{8}{\_ Metric}} = {{Smoothed\_ L}_{1}{\_ E}_{2}}}} \\{\quad {{{Direction}_{9}{\_ Metric}} = {{Smoothed\_ C}_{1}{\_ E}_{2}}}}\end{matrix} \right. & \text{(2B)}\end{matrix}$

where αε[0,1] is the forgetting factor.

Furthermore, in accordance with a preferred embodiment of the presentinvention, the step of assigning metrics to all possible directions canalso include a comparison of the strongest metric in Equations (1) orEquations (2B) to the second strongest metric or some other metric. Ifthe difference is smaller than a certain pre-specified threshold“Metric_Threshold”, Direction_(3—)Metric is assigned a large value. Thisensures that in such a case, Direction_(3—)Metric is chosen in step 12and no finger movement occurs (see FIG. 1). Thus, finger movement occursonly when the strongest direction metric is larger than the comparedmetric by more than Metric_Threshold.

In another preferred embodiment, after a finger movement occurs in step14, all smoothed direction metrics in Equations (2A) are zeroed, so asto abandon all measurements related to the previous finger locations.

It will be apparent to someone skilled in the art that various otherdirection metrics can be assigned based on the channel tap estimator,reception quality and their combination with finger power measurements.

When one or more fingers are moved in step 14, the relationships of allthe fingers being tracked change. It is possible that a finger is nowsufficiently far away to no longer be part of the block, or,alternatively, that a finger not of the block is now close to one of thefingers of the block and, thus, must be included in the block. Theseconsiderations are performed in step 16 and the finger block(s) areredefined, if necessary.

The present invention can be implemented for any number of fingers in afinger block and for any definition of a finger block. One exemplarydefinition is that a finger block has two fingers, and RANGE accepts thesingle value RANGE=⅞ Tc. Thus, the two fingers in the finger block areexactly ⅞ Tc apart and are not allowed to become closer. Therefore,directions 6, 8 and 9 are discarded. This reduces the number of possiblemovements from nine to six, as shown in FIG. 4, to which reference isnow made. The two fingers can become late (direction1), become early(direction2), or stay at the correct sampling time (direction3). Thesethree movements do not alter the timing relationship between the fingersin the block. The three other movement directions cause a finger blockto separate, are considered where the later finger becomes later(direction4), the earlier finger becomes earlier (direction5) or theearlier finger becomes earlier and the later finger becomes later(direction7).

According to a preferred embodiment of the present invention, the numberof possible movements can be further reduced to five, where the fivedirections are directions 1-5 of FIG. 4.

FIG. 5 illustrates a joint tracking method for a finger block of twofingers using the movements shown in FIG. 4. FIG. 5 is a special case ofthe general flow chart of FIG. 3. Only those fingers which are close toeach other, are jointly tracked. The other fingers are individuallytracked by conventional methods. The individual tracking will not bediscussed herein as it is well known in the art.

In step 30, the early, late and central power measures PowerEarly_(i),PowerLate_(i), and PowerCenter_(i), respectively, of each finger arederived (i is a number of finger).

In step 32, the direction metrics related to the six possible movementsof FIG. 4, are determined in accordance with Eqs. 2.a and 2.b. Thedirection metrics are formed from the relevant combinations of the pairsof the individual early, late and central powers. The pairs are: bothearly (PowerEarly₁, PowerEarly₂), both late (PowerLate₁, PowerLate₂),both central (PowerCenter₁, PowerCenter₂), early-central (PowerEarly₁,PowerCenter₂), central-late (PowerCenter₁, PowerLate₂) and early-late(PowerEarly₁, PowerLate₂), and the calculations are as follows:

Smoothed_E_(1—)E₂=(1−α)Smoothed_E_(1—)E₂+α(PowerEarly₁+PowerEarly₂)

Smoothed_L_(1—)L₂=(1−α)Smoothed_L_(1—)L₂+α(PowerLate₁+PowerLate₂)

Smoothed_E_(1—)C₂=(1−α)Smoothed_E_(1—)C₂+α(PowerEarly₁+PowerCenter₂)

Smoothed_C_(1—)L₂=(1−α)Smoothed_C_(1—)L₂+α(PowerCenter₁+PowerLate₂)

Smoothed_C_(1—)C₂=(1−α)Smoothed_C_(1—)C₂+α(PowerCenter₁+PowerCenter₂)

Smoothed_E_(1—)L₂=(1−α)Smoothed_E_(1—)L₂+α(PowerEarly₁+PowerLate₂)  (3)

In step 34, the largest direction metric is selected, and in step 36, itis checked whether the maximal direction metric involves fingermovement. If the maximal direction metric is the central metricSmoothed_C_(1—)C₂, no finger is moved. Otherwise, in step 38, it ischecked whether the maximal metric is larger than another metric by morethan a predetermined threshold, for example the value 16. If the maximalmetric is sufficiently large, then in step 40 the relevant fingers areadjusted, the metrics are zeroed, and joint tracking is switched off ifthe distance between the fingers is more than RANGE. Finally, the fingerblocks are redefined (step 42).

The threshold check of step 38 compares the maximal metric with anothermetric. If the maximal metric is the double early metricSmoothed_E_(1—)E₂, indicating that both fingers have become earlier(direction 2 of FIG. 4), then it is compared to the double late metricSmoothed_L_(1—)L₂. If the difference Smoothed_E_(1—)E₂−Smoothed_L_(1—)L₂is greater than the threshold, the finger adjustment of step 40 is theadvancement of both finger_1 and finger_2. Since the fingers were moved,new finger blocks are defined based on the new delay of the fingers.

If the maximal metric is the double late metric Smoothed_L_(1—)L₂,indicating that both fingers have become later (direction 1 of FIG. 4),then it is compared to the double early metric Smoothed_E_(1—)E₂. If thedifference Smoothed_L_(1—)L₂−Smoothed_E_(1—)E₂ is greater than thethreshold, the finger adjustment of step 40 is the delay of bothfinger_1 and finger_2. Since the fingers were moved, new finger blocksare defined based on the new delay of the fingers.

If the maximal metric is the mixed metric Smoothed_E_(1—)C₂, indicatingthat the first finger has become earlier (direction 5 of FIG. 4), thenit is compared to the central metric Smoothed_C_(1—)C₂. If thedifference Smoothed_E_(1—)C₂−Smoothed_C_(1—)C₂ is greater than thethreshold, the finger adjustment of step 40 is the advancement offinger_1. Since finger_1 moved away, the two fingers are nowsufficiently separate and the joint tracking is now switched off. Sincethe fingers were moved, new finger blocks are defined based on the newdelay of the fingers.

Similarly, if the maximal metric is the mixed metric Smoothed_C_(1—)L₂,indicating that the second finger has become later (direction 4 of FIG.4), then it is compared to the central metric Smoothed_C_(1—)C₂. If thedifference Smoothed_C_(1—)L₂−Smoothed_C_(1—)C₂ is greater than thethreshold, the finger adjustment of step 40 is the delay of finger_2.Since finger_2 moved away, the two fingers are now sufficiently separateand the joint tracking is now switched off. Since the fingers weremoved, new finger blocks are defined based on the new delay of thefingers.

If the maximal metric is the mixed metric Smoothed_E_(1—)L₂, indicatingthat the first finger has become earlier and the second finger hasbecome later (direction 7 of FIG. 4), then it is compared to the centralmetric Smoothed_C_(1—)C₂, If the differenceSmoothed_E_(1—)L₂−Smoothed_C_(1—)C₂ is greater than the threshold, thefinger adjustment of step 40 is the advancement of finger_1 and thedelay of finger_2. Since the fingers have moved apart, the two fingersare now sufficiently separate and the joint tracking is now switchedoff. Since the fingers were moved, new finger blocks are defined basedon the new delay of the fingers.

Reference is now made to FIG. 6, which details the operations of step 42(redefine finger blocks). Each finger FINGER_X which has been moved isreviewed to consider whether or not it is now close to other fingers. Instep 60, the delays between FINGER_X and all the other fingers not inthe same finger block with FINGER_X are determined, and, in step 62, thefinger closest to FINGER_X is labeled FINGER_Y. The delay betweenFINGER_X and FINGER_Y is labeled MinDelay.

The delay MinDelay is then compared (step 63) to a minimum allowedthreshold delay, for example ⅞ Tc. If the delay MinDelay is larger thanthe threshold level, then the finger is sufficiently far from the otherfingers to be individually tracked. Thus, conventional early-late timetracking will be performed (step 64) on it. Otherwise, FINGER_X andFINGER_Y are too close to each other and either one of them has to beremoved or they both must be tracked as part of a finger block.

In step 65 FINGER _X and FINGER_Y are checked to see if they are part offinger block. If FINGER_X and FINGER_Y are not part of a finger block, afinger block is created (step 69) from FINGER_X and FINGER_Y. Otherwise,in step 66, FINGER_X is checked to see if it is part of a finger block.If FINGER_X is part of the block, FINGER_Y is switched off (i.e. it isno longer used as a finger) in step 67. Otherwise, FINGER_X is switchedoff in step 68. It will be appreciated that if the switched off fingeris part of the block having a pair of fingers, joint tracking is alsoswitched off.

It will be appreciated that the presence of a finger block affectsvarious other elements of the receiver, all of which must be slightlymodified to include operation with finger blocks. FIGS. 7 and 8, towhich reference is now made, illustrate these modified operations forclose path detection, finger replacement and finger removal.

Reference is now made to FIG. 7 which details the method in which it isdecided whether a new path candidate should be added to the rake as anew finger. The method for generating new path candidates is known inthe art and is therefore not described. In step 70 CompositeFingerPoweris computed for all active rake fingers, where

CompositeFingerPower=F(PowerCenter_(i), ρ, Δ)

and ρ is the crosscorrelation between the finger and its closestneighbor in the rake receiver or between the finger to the new fingercandidate, if it is closer than all other rake fingers, Δ is the delayto the closest finger, i is the finger number, and F(.,.,.) is anarbitrary function. For example, if Δ>=T_(c) F(PowerCenter_(i), ρ, Δ)can be set equal to PowerCenter, i.e.

F(PowerCenter_(i), ρ, Δ)=PowerCenter_(i)  (4)

in which case the method in FIG. 7 reduces to the conventional fingerassignment method that assigns fingers based only on their relativepower.

The crosscorrelation ρ can be one of the following $\begin{matrix}{{\rho = \frac{{Re}{\sum\limits_{k = 1}^{N}\left( {{{ch}_{{Finger}_{i}}(k)} \cdot {{ch}_{{Finger}_{i}}^{*}(k)}} \right)}}{\sum\limits_{k = 1}^{N}\left| \left( {{{ch}_{{Finger}_{i}}(k)} \cdot {{ch}_{{Finger}_{i}}^{*}(k)}} \right) \right|}}{or}} & (5) \\{\rho = \frac{\left| {\overset{N}{\sum\limits_{k = 1}}{{{ch}_{{Finger}_{i}}(k)} \cdot {{ch}_{{Finger}_{i}}^{*}(k)}}} \right|}{\sqrt{\sum\limits_{k = 1}^{N}\left| {{ch}_{{Finger}_{i}}(k)} \middle| {}_{2}{\cdot \sum\limits_{k = 1}^{n}} \middle| {{ch}_{{Finger}_{i}}^{*}(k)} \right|^{2}}}} & (6)\end{matrix}$

where ch_(Finger1)(k), is the channel estimate for Finger_(i),Finger_(j) is the finger closest to Finger_(i), (.)* stands for thecomplex conjugate of the bracketed expression and the sum is over apre-specified length of time.

If Δ<T_(c), F(PowerCenter_(i), ρ, Δ) can be defined as:

F(PowerCenter_(i), ρ, Δ)=PowerCenter_(i)*ƒ(ρ)  (7)

where the function f(ρ) is any function, for example:

Case Function f(ρ) ρ < 0.35 1.0  0.35 < ρ < 0.46 0.9  0.45 < ρ < 0.550.8  0.55 < ρ < 0.65 0.66 0.65 < ρ < 0.75 0.52 0.75 < ρ < 0.85 0.38 0.85< ρ < 0.92 0.23 0.92 < ρ < 1.0  0.14

In step 72, a similar calculation is performed for the new fingercandidate and CompositeFingerCandidatePower is computed. It will beappreciated that function F(.,.,.) and ρ calculation can be differentfor step 70 and step 72, and other choices of function can be used, e.g.ρ can be calculated by incorporating an exponential forgetting factor.

In step 73, two different branches are examined. If the number offingers currently used equals the maximum number allowed (i.e. all rakefingers are active), first branch have to be executed in this branchCompositeFingerCandidatePower is compared (step 77) with the minimum ofall CompositeFingerPower's. If it is greater than this minimal power,then the finger, associated with minimal CompositeFingerPower, is movedto the location (timing) of the new finger candidate (step 78). Finally,in step 79, a redefinition of the finger blocks is taking place. IfCompositeFingerCandidatePower is less than this minimal power, nofingers are moved.

If the number of fingers currently used is less than the maximum numberallowed, CompositeFingerCandidatePower is compared (step 74) to athreshold NewFingerThreshold. If it is greater than the threshold, thena new finger is added (step 75) and a redefinition of the finger blocksis taking place (step 76). Otherwise, the new finger candidate is notadded to the rake receiver.

FIG. 8 illustrates the removal of an existing finger from a fingerblock, comprised from two fingers, such as might be periodicallyperformed. The method of FIG. 8 is performed periodically to removefingers. The conventional method for switching off a finger is to do sowhen the finger power measurements are low relative to other fingers.This criterion is not effective for removing a finger from a fingerblock. The present invention uses the following FingerCost function toevaluate whether finger in finger block should be turned off:

FingerCost=α·min(PowerCenter₁,PowerCenter₂ )−ρ·C  (8)

where ρ is any desired value between zero and one, PowerCenter₁ andPowerCenter₂ are powers of fingers in finger block C is a prespecifiedconstant and ρ is defined in Equation (5) and Equation (6) with i=1 andj=2.

As shown in FIG. 8, initially, the correlation ρ between the two fingersfinger_1 and finger_2 is determined (step 80) after which the value ofFingerCost is calculated (step 82) and compared (step 84) to athreshold.

If the result is less than the threshold, then the finger having theminimum average power is removed (step 86) and the joint trackingoperation is stopped (since there remains only one finger in fingerblock). Otherwise, no fingers are removed. A similar finger removermechanism can be defined for finger blocks having three or more fingers.

It will be appreciated by persons skilled in the art that the presentinvention is not limited by what has been particularly shown anddescribed herein above rather the scope of the invention is defined bythe claims that follow.

What is claimed is:
 1. A method comprising: measuring powers of at least one finger of a finger block comprising two or more fingers; calculating crosscorrelations of the fingers of said finger block with one another; and deactivating one of the fingers of said finger block when a function of said powers and said crosscorrelations satisfies a predetermined criterion.
 2. A method according to claim 1, wherein said finger block has two fingers.
 3. A method according to claim 2, wherein said function is FingerCost=α·min(PowerCenter₁,PowerCenter₂)−ρ·C where α is a value between zero and one, PowerCenter₁ and PowerCenter₂ are powers of fingers in said finger block, C is a predetermined value and ρ is one of said calculated crosscorrelations.
 4. A method according to claim 3, wherein ρ is $\rho = {\frac{{Re}\quad {\sum\limits_{k = 1}^{N}\left( {{{ch}_{Finger1}(k)} \cdot {{ch}_{Finger2}^{*}(k)}} \right)}}{\sum\limits_{k = 1}^{N}{\left( {{{ch}_{Finger1}(k)} \cdot {{ch}_{Finger2}^{*}(k)}} \right)}}.}$


5. A method according to claim 3, wherein ρ is $\rho = {\frac{{\sum\limits_{k = 1}^{N}{{{ch}_{Finger1}(k)} \cdot {{ch}_{Finger2}^{*}(k)}}}}{\sqrt{\sum\limits_{k = 1}^{N}{{{{ch}_{Finger1}(k)}}^{2} \cdot {\sum\limits_{k = 1}^{n}{{{ch}_{Finger2}^{*}(k)}}^{2}}}}}.}$


6. A method according to claim 1, wherein calculating said crosscorrelations comprises using a forgetting factor.
 7. A method comprising: calculating a crosscorrelation of a finger candidate which is an inactive finger and an active finger whose time delay is closest to the time delay of said finger candidate; and assigning said finger candidate to a new oath when a composite finger power for said finger candidate satisfies a predetermined criterion, wherein said composite finger power is based at least in part upon said crosscorrelation.
 8. A method according to claim 7, wherein calculating said crosscorrelation comprises using a crosscorrelation function defined as $\rho = {\frac{{Re}\quad {\sum\limits_{k = 1}^{N}\left( {{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}} \right)}}{\sum\limits_{k = 1}^{N}{\left( {{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}} \right)}}.}$


9. A method according to claim 7, wherein calculating said crosscorrelation comprises using a crosscorrelation function defined as $\rho = {\frac{{\sum\limits_{k = 1}^{N}{{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}}}}{\sqrt{\sum\limits_{k = 1}^{N}{{{{ch}_{FingerCandidate}(k)}}^{2} \cdot {\sum\limits_{k = 1}^{n}{{{ch}_{ClosestFinger}^{*}(k)}}^{2}}}}}.}$


10. A method according to claim 7, wherein calculating said crosscorrelation comprises using a forgetting factor.
 11. An apparatus comprising: a finger power measurement unit to measure powers of at least one finger of a finger block comprising two or more fingers; a correlation calculator to calculate crosscorrelations of the fingers of said finger block with one another; and a deactivation unit to deactivate one of the fingers of said finger block when a function of said powers and said crosscorrelations satisfies a predetermined criterion.
 12. An apparatus according to claim 11, wherein said finger block has two fingers and said function is FingerCost=α·min(PowerCenter₁,PowerCenter₂)−ρ·C where α is a value between zero and one, PowerCenter₁ and PowerCenter₂ are powers of fingers in the finger block, C is a predetermined value and ρ is one of the calculated crosscorrelations.
 13. An apparatus according to claim 12, wherein ρ is $\rho = {\frac{{Re}\quad {\sum\limits_{k = 1}^{N}\left( {{{ch}_{Finger1}(k)} \cdot {{ch}_{Finger2}^{*}(k)}} \right)}}{\sum\limits_{k = 1}^{N}{\left( {{{ch}_{Finger1}(k)} \cdot {{ch}_{Finger2}^{*}(k)}} \right)}}.}$


14. An apparatus according to claim 12, wherein ρ is $\rho = {\frac{{\sum\limits_{k = 1}^{N}{{{ch}_{Finger1}(k)} \cdot {{ch}_{Finger2}^{*}(k)}}}}{\sqrt{\sum\limits_{k = 1}^{N}{{{{ch}_{Finger1}(k)}}^{2} \cdot {\sum\limits_{k = 1}^{n}{{{ch}_{Finger2}^{*}(k)}}^{2}}}}}.}$


15. An apparatus according to claim 11, wherein said correlation calculator uses a forgetting factor.
 16. An apparatus comprising: a correlation calculator to calculate a crosscorrelation of a finger candidate which is an inactive finger and an active finger whose time delay is closest to the time delay of said finger candidate; and an assignment unit to assign said finger candidate to a new path when a composite finger power for said finger candidate satisfies a predetermined criterion, wherein said composite finger power is based at least in part upon said crosscorrelation.
 17. An apparatus according to claim 16, wherein said crosscorrelation calculator uses a crosscorrelation function defined as $\rho = {\frac{{Re}\quad {\sum\limits_{k = 1}^{N}\left( {{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}} \right)}}{\sum\limits_{k = 1}^{N}{\left( {{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}} \right)}}.}$


18. An apparatus according to claim 16, wherein said crosscorrelation calculator uses a crosscorrelation function defined as $\rho = {\frac{{\sum\limits_{k = 1}^{N}{{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}}}}{\sqrt{\sum\limits_{k = 1}^{N}{{{{ch}_{FingerCandidate}(k)}}^{2} \cdot {\sum\limits_{k = 1}^{n}{{{ch}_{ClosestFinger}^{*}(k)}}^{2}}}}}.}$


19. An apparatus according to claim 16, wherein said correlation calculator uses a forgetting factor.
 20. A method comprising: calculating a crosscorrelation of a finger candidate which is an active finger and another active finger whose time delay is closest to the time delay of said finger candidate; and reassigning said finger candidate to a new path when a composite finger power for said finger candidate satisfies a predetermined criterion, wherein said composite finger power is based at least in part upon said crosscorrelation.
 21. A method according to claim 20, wherein calculating said crosscorrelation comprises using a crosscorrelation function defined as $\rho = {\frac{{Re}\quad {\sum\limits_{k = 1}^{N}\left( {{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}} \right)}}{\sum\limits_{k = 1}^{N}{\left( {{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}} \right)}}.}$


22. A method according to claim 20, wherein calculating said crosscorrelation comprises using a crosscorrelation function defined as $\rho = {\frac{{\sum\limits_{k = 1}^{N}{{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}}}}{\sqrt{\sum\limits_{k = 1}^{N}{{{{ch}_{FingerCandidate}(k)}}^{2} \cdot {\sum\limits_{k = 1}^{n}{{{ch}_{ClosestFinger}^{*}(k)}}^{2}}}}}.}$


23. A method according to claim 20, wherein calculating said crosscorrelation comprises using a forgetting factor.
 24. An apparatus comprising: a correlation calculator to calculate a crosscorrelation of a finger candidate which is an active finger and another active finger whose time delay is closest to the time delay of said finger candidate; and an assignment unit to reassign said finger candidate to a new path when a composite finger power for said finger candidate exceeds the minimum composite finger power of all active fingers, wherein said composite finger power is based at least in art upon said crosscorrelation.
 25. An apparatus according to claim 24, wherein said crosscorrelation calculator uses a crosscorrelation function defined as $\rho = {\frac{{Re}\quad {\sum\limits_{k = 1}^{N}\left( {{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}} \right)}}{\sum\limits_{k = 1}^{N}{\left( {{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}} \right)}}.}$


26. An apparatus according to claim 24, wherein said crosscorrelation calculator uses a crosscorrelation function defined as $\rho = {\frac{{\sum\limits_{k = 1}^{N}{{{ch}_{FingerCandidate}(k)} \cdot {{ch}_{ClosestFinger}^{*}(k)}}}}{\sqrt{\sum\limits_{k = 1}^{N}{{{{ch}_{FingerCandidate}(k)}}^{2} \cdot {\sum\limits_{k = 1}^{n}{{{ch}_{ClosestFinger}^{*}(k)}}^{2}}}}}.}$


27. An apparatus according to claim 24, wherein said crosscorrelation calculator uses a forgetting factor. 